An emerging concept in biology assigns the amino acid tryptophan specific roles at the membrane/water interface that help to determine the conformation and biological function of membrane-spanning proteins. Previous studies involving the antibiotic model system gramicidin A (gA) have illustrated the importance of the indole ring of tryptophan (Trp) in anchoring proteins to a bilayer membrane and promoting ionic currents. To further investigate these phenomena, derivatives of Trp that have lost hydrogen-bonding ability (1-methyl-Trp ), have an altered dipole moment (7-aza-Trp), or both (1-methyl-7-azaTrp) were chosen for incorporation into gA. Gramicidin analogues that incorporate these modified Trps were then analyzed by single channel experiments. In addition, methods were developed for the selective exchange of indole hydrogen with deuterium (a heavy isotope of hydrogen, 2 H) using a Raney nickel catalyst. The 2 H labels enable determinations of the orientation of each Trp indole ring with respect to the membrane surface using solid-state deuterium NMR spectroscopy. The last method 1 pursued involves the application of ab initio molecular modeling programs to calculate the side-chain dipole moments of Trp, 1-methyl-Trp, 7-aza-Trp, and 7-aza-1-methyl-Trp. Therefore, this project combines both experimental and theoretical aspects of scientific research. Results from the single-channel experiments of [7-azaTrp 1 and [7-aza-1-methyl-Trp 1 gA analogues indicate that there is a positive correlation between channel conductance and the magnitude of the side chain dipole moments. A new methodology involving a Raney nickel catalyst was also successfully developed that allows for -75% of7-aza-Trp 's sixth hydrogen to exchange with deuterium. In addition to these experimental results, the ab initio program PQS was used to generate theoretical predictions of Trp side chain dipoles that were comparable to experimentally determine dipoles, and that allowed for the calculation of 1- methyl-Trp's side chain dipole. Beyond the immediate results, the more general implication of this project is the fundamental knowledge gained concerning the interactions of Trp with other amino acids, water, and lipids. These studies will contribute to a better understanding of folded proteins- especially those that span biological membranes.
Scherer, E. M. (2002). Properties of Modified Tryptophans in a Membrane- Spanning Channel. Inquiry: The University of Arkansas Undergraduate Research Journal, 3(1). Retrieved from https://scholarworks.uark.edu/inquiry/vol3/iss1/20